Impact of heat exposure during immobilization‐induced de‐training and re‐training on aerobic capacity and haemoglobin mass

The aim of this work was to assess the effect of heat exposure on cardiorespiratory and haematological responses during de-training and re-training. Nineteen men (33.8 ± 2.7 years; 182 ± 5.7 cm, 84.4 ± 9.3 kg) completed 4 weeks of pre-training followed by heat exposure (HEAT; n = 9) or control (CON; n = 10). Both groups then de-trained for 2 weeks with lower-limb immobilization followed by 2 weeks of re-training. Cardiorespiratory fitness and total haemoglobin mass (Hb_mass) were measured at baseline (BASE), prior to (IMMO_pre) and following (IMMO_post) immobilization and after a 'return-to-sport' (RTS_post) training phase. Compared with IMMO_pre, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> at the gas exchange threshold (GET) was reduced (d = -0.53; P < 0.005) at IMMO_post, whereas maximal oxygen uptake ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}{\mathrm{max}}}}$</mml:annotation></mml:semantics> </mml:math> ) did not change significantly (d = 0.14; P = 0.07). The reduction in GET <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> was more pronounced for HEAT than CON (d = -0.77; P = 0.001). At IMMO_post, GET and peak power output were lower than IMMO_pre (d = -1.4, P < 0.005 and d = 0.43, P = 0.01, respectively), however there was no difference between groups. At RTS_post, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> at GET increased again in both groups yet remained lower than IMMO_pre although the reduction in HEAT (d = -0.42; P = 0.43) was less than CON (d = -0.8; P = 0.05). At RTS_post, HEAT fully recovered GET power losses compared to IMMO_post (d = 1.1; P = 0.001), while CON only showed a trivial change (d = 0.07; P = 0.1). No significant changes were observed in Hb_mass, haematocrit or plasma volume throughout the study (P ≥ 0.763). Heat exposure did not attenuate a decline in cardiorespiratory fitness during immobilization-induced de-training, but could potentiate its recovery upon re-commencement of training.